In this paper, performance of the antenna system is analyzed, as the clock frequency in the external digital circuit is increased. To verify the interrelation between antenna performance and digital clock, inverted-F antenna system and the mixed signal circuit are designed. Besides, the coupled noises to the antenna system are analyzed as the operating frequency of the digital clock is increased by 0.5, 1, and 2 GHz. Hence, the peak to peak coupled noises to the antenna system are 1.169, 3.386, and 18.034 mV for periodic digital clocks, and 2.054, 6.316, and 17.379 mV for random digital clocks, respectively. Therefore, to reduce the coupled noise to the antenna system due to the digital clocks, several methods are proposed; anti-resonance structure, 180° delay line, and LC filter. As a result, maximum ratio of the noise reduction is achieved about 78% by applying 180° delay line.